The invention relates to magnesium boride superconductors. In particular, it relates to the processing of magnesium boride into superconducting wires.
Although magnesium boride (MgB2), a hexagonal, layered compound, has been known for years, its superconducting properties have only been recently discovered by J. Akimitsu et al. (Symposium on Transition Metal Oxides, Sendai, Japan, Jan. 10, 2001). The recent discovery of superconductivy at about 39K has produced a high level of activity directed to characterizing MgB2 in more detail and to synthesizing the superconductor in bulk form. MgB2 behaves like a classic BSC superconductor with a relatively low irreversibility field. MgB2 is an interesting superconducting material due to its strongly linked current flow, even though it has a relatively low Hc2(0) and only a modest critical temperature, Tc. The irreversibility field parallel to the c-axis is between 2 and 4 T at 25 K, and therefore MgB2 will be bested suited for applications at operating temperature and field ranges of less than about 30 K (e.g., 15 to 30K) and less than about 3 T (e.g., 0–3T), respectively. Both monofilament and multifilamentary wires are attractive additions to the available superconducting wires. Multifilament wire desirably is capable of being twisted and cabled.
Takano et al. prepared bulk samples by hot pressing, and found considerable differences with sintering temperatures between 775° C. and 1000° C. (Preprint). Transitions were much sharper in the sample pressed at 1000° C. than the one pressed at 775° C., and the normal state resistivity was much lower. The M-H curves at 10K through 35K also showed much higher critical currents for the 1000° C. sample. Critical current densities (Jc) derived from these M-H curves were typically an order of magnitude lower than those in the powder and were 400 A/mm2 at 20K, 1T. The upper critical field was estimated to be over 25T.
MgB2 is typically formed by heating magnesium and boron in a sealed tantalum-lined ampoule at high temperatures (995° C.) (Bud'ko et al., Preprint; and Bianconi et al., Preprint). Takano et al. prepared bulk samples by hot pressing, and found considerable differences with sintering temperatures between 775° C. and 1000° C. (Preprint, Mar. 9, 2001, xxx.lanl.gov/abs/cond-mat). Liquid magnesium is chemically aggressive and will react with almost any oxide due to the high stability of MgO. Lower reaction temperatures are desired to reduce reaction of the reactive components with their environment.
Among the more useful known low temperature superconductor (LTS) materials is Nb3Sn, an intermetallic compound having the so-called A-15 crystal structure. Both intermetallic and ceramic high temperature superconductor (HTS) superconductors perform better when the superconductive material is divided among a number of filaments embedded in a metallic matrix. LTS and HTS materials have been prepared as multifilamentary conductors. Multifilamentary wires are particularly useful at low temperature or to reduce ac losses. At higher temperature, i.e., ≧20° K, a monofilament wire can sometimes be used.
A typical process for the manufacture of a multifilamentary Nb3Sn conductor begins with the drilling of a plurality of holes in a Cu/Sn bronze billet for the insertion of Nb rods. This billet is then extruded to a rod, drawn down to fine wire, and then heated to form the superconductor. A higher filament count is achieved by cutting the rod prior to drawing into a large number of equal lengths at some intermediate size, inserting these into an extrusion can, extruding this assembly and drawing the resultant billet into a wire, which is then heated to form the superconductor. The rod may be drawn through a hex-shaped die prior to cutting, which provides a space filling shape for subsequent assembly.
Mechanical alloying of constituent metals of a superconducting material also is known. Mechanical alloying has long been known and was originally developed for the manufacture of high strength structural alloys. Mechanical alloying has been used for the production of low temperature superconducting Nb3Sn and Nb3Al powders. See Larson et al., Manufacture of Superconducting Materials, Proc. Intl. Conf. November 1976, Ed. R. W. Meyerhoff, p. 155 (1976).
In the field of HTS, mechanical alloying has been used to make so-called metallic precursor filaments in a metallic matrix, which can be shaped in the metallic state and then transformed into the HTS ceramic oxide wire after completion of the extrusion and wire drawing. For example, suitable metal powders are milled into a fine metallic powder that is used to fill silver tubes that are then processed by extrusion or drawing into filaments. These are then bundled in a silver tube, and extruded again to make a multi-filamentary wire, if desired. The HTS phase is formed by oxidizing the metallic precursor filaments. Transmission electron microscopy of the metallic precursor powders has shown that these are not layered but amorphous, with no discernable or very fine grained multiphase crystalline structure. The elements are often well mixed on an atomic scale. See, Otto et al., IEEE Trans. Appl. Supercond. 3(1):915 (1993); and Yurek et al. Met. Trans., 18A:1813 (1987).
Little is known or understood about the superconducting properties and related processing capabilities for the newly identified superconductor MgB2. Methods of forming magnesium boride precursor powders, of obtaining long lengths of magnesium boride superconductor wires or tapes and of forming magnesium boride films are desired.